Landscape‐dependent time versus energy optimizations in pelicans migrating through a large ecological barrier

Abstract

During migration, birds are often forced to cross ecological barriers, facing challenges due to scarcity of resources and suitable habitats. While crossing such barriers, birds are expected to adjust their behaviour to reduce time, energy expenditure and associated risks. We studied the crossing of the Sahara Desert by the Great White Pelican (Pelecanus onocrotalus), a large wetland‐specialist. We focused on decisions made by migrating pelicans along different parts of the southbound autumn migration, their response to local environmental conditions and the implications for time and energy optimizations. We compared the observed pelicans' migration routes with simulated ‘direct‐pass’ (shortest, mostly across the desert) and ‘corridor‐pass’ (along the Nile River) routes, and used GPS, body acceleration and atmospheric modelling to compare flight behaviour along the Nile River versus the desert. The observed route was significantly shorter and faster than the simulated corridor‐pass route and not significantly different from the simulated direct‐pass one. Daily flights over the desert were longer than along the Nile River, with flying time extending to late hours of the day despite unfavourable atmospheric conditions for soaring–gliding flight. Moreover, the pelicans' behavioural response to atmospheric conditions changed according to the landscape over which they flew. Overall, the pelicans showed stronger behavioural adjustments to atmospheric conditions over the desert than along the Nile River. Our findings suggest that migrating pelicans primarily acted as time minimizers while crossing the Sahara Desert, whereas energetic optimization was only considered when it did not substantially compromise time optimization. The pelicans took the almost shortest possible route, only following the Nile River along its south‐oriented parts, and frequently staged overnight in the desert far from water, despite being large, wet‐habitat specialists. Correspondingly, their behavioural response to atmospheric conditions changed according to the landscape over which they were flying, switching between time (over the desert) and energy (over the Nile River) optimization strategies. Our results suggest that the interaction between landscape and atmospheric conditions depict a flexible, yet primarily time‐dominated, migration optimization strategy. A free Plain Language Summary can be found within the Supporting Information of this article.

Causes and consequences of facultative sea crossing in a soaring migrant

“…Still, how migration route is influenced by atmospheric and geographical factors is much less clear. Route selection over ecological barriers such as large waterbodies may depend on weather and geographical features (Alerstam, ; Becciu et al, ; Efrat, Hatzofe, & Nathan, ; Eisaguirre et al, ; Nourani, Yamaguchi, Manda, & Higuchi, ), affecting migration time and energy expenditures, with consequences for animal fitness (Shamoun‐Baranes, Bouten, & Van Loon, ; Shamoun‐Baranes et al, ). Large terrestrial soaring birds depend on local atmospheric conditions during their flight, since they utilize thermal uplifts to gain height and later glide towards their destination (Norberg, ).…”

“…Still, how migration route is influenced by atmospheric and geographical factors is much less clear. Route selection over ecological barriers such as large waterbodies may depend on weather and geographical features (Alerstam, 2001;Becciu et al, 2019;Efrat, Hatzofe, & Nathan, 2019;Eisaguirre et al, 2018;Nourani, Yamaguchi, Manda, & Higuchi, 2016), affecting migration time and energy expenditures, with consequences for animal fitness (Shamoun- Baranes, Bouten, & Van Loon, 2010;Shamoun-Baranes et al, 2017).…”

“…These areas can be regarded as 'high transport cost' areas (Alerstam, 2001). We note that differences in transport cost may be caused by additional factors, such as variable wind conditions experienced by the birds when travelling over different areas (Alerstam, 2001;Efrat et al, 2019). Besides increasing the transport cost, barrier crossing versus barrier circumvention (i.e.…”

“…Besides increasing the transport cost, barrier crossing versus barrier circumvention (i.e. facultative barrier crossing) may shorten migration distance, with possible consequences for migration time saving (Alerstam, 2001;Efrat et al, 2019).…”

Differential survival throughout the full annual cycle of a migratory bird presents a life‐history trade‐off

“…The apparently higher survival during the Sahara crossing of Egyptian vultures as compared to other birds may be attributable to species‐specific behavioural adaptations (e.g. Efrat et al., 2019). Thus, what constitutes a natural barrier that incurs a significant survival cost on migration is likely species‐specific.…”